GeSbTe, a ternary compound of Germanium, Antimony and Tellurium, also known as GST, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications.
A characteristic that makes GST useful as a phase-change memory is its ability to effect a reversible phase change when heated or cooled, switching between stable high resistance amorphous phase to low resistance crystalline phase in nanosecond-timescale. GST memory has many desirable qualities such as better scaling quality, fast read/write speed, strong cycling performance, compatibility with current CMOS logic process, non-volatility, endurance of more than 1013 read-write cycles, non-destructive read, direct overwriting, and data retention time of more than 10 years.
A typical phase-change memory device includes layers such as a top electrode, a GST layer, a bottom electrode and other dielectric layers. Production of a phase-change memory device is similar to production of a typical integrated circuit, which involves sequential deposition of conductive, semiconductive, or insulative layers on a substrate, such as a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed.
A GST layer, for example, can be deposited on a patterned insulative layer to fill holes in the insulative layer. After planarization, the portions of the GST layer remaining between the raised patterns of the insulative layer form plugs that provide the memory cells on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing liquid, such as a slurry with abrasive particles, is supplied to the surface of the polishing pad. The substrate surface is then polished by the moving polishing pad until an end point is called.
In order to determine the effectiveness of a polishing operation, a “blank” substrate (e.g., a wafer with multiple layers but no pattern) or a test substrate (e.g., a wafer with the pattern to be used for device wafers) is polished in a tool/process qualification step. After polishing, the substrate is removed from the polishing system and the remaining layer thickness (or another substrate property relevant to circuit operation, such as conductivity) is measured at several points on the substrate surface using an in-line or stand-alone metrology station. The variation in layer thickness provide a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. Polishing parameters, such as polishing time and polishing load, can be adjusted in subsequent polishing operations based on the resulting metrology results.
GST has two stable structural phase states, crystalline and amorphous. In its stable state, crystalline GST has two possible configurations: hexagonal and a metastable face centered cubic (FCC) lattice. When GST is rapidly crystallized, it can also have a distorted rocksalt structure. GST also has many vacancies in the lattice, ranging from 20% to 25% depending on the specific GST compound.
Existing in-line or stand-alone metrology station using ellipsometry or X-ray reflectometry (XRR) can provide accurate and reliable thickness measurements (e.g., using ellipsometry) and precise positioning of a sensor to desired measurement locations on the substrate. Structural phase can be determined using X-ray diffraction methods.